An adverse drug reaction (ADR) occurs when a medication has an unexpected, unwanted, and harmful effect on the patient, even when the medication has been prescribed and taken correctly — based on standardized guidelines. While some adverse drug reactions are caused by drug-drug interactions or allergic responses, the majority of reactions appear to be caused by genetic factors. And while normal genetic variation can cause an ADR in some patients, it can result in a medication having no effect in others.When precision medicine fully replaces conventional medicine and its familiar tendency towards crisis management, no two individuals will be treated the same even when their diagnoses are identical.